ovsdb-idl: Make selecting tables and columns to replicate more flexible.

ovsdb-idl: Make selecting tables and columns to replicate more flexible.

Until now, by default the IDL replicated all tables and all columns in the
database, and a few functions made it possible to avoid replicating
selected columns.  This commit adds a mode in which nothing is replicated
by default and the client code is responsible for specifying each column
and table that it is interested in.  The following commit adds a user for
this mode.